1. Field of the Invention
The invention relates to a fuel cell, and more particularly, to a gas flow path structure of a fuel cell. The invention also relates to a manufacturing method of expanded metal used in the fuel cell.
2. Description of Related Art
A fuel cell is formed as a stacked structure in which a plurality of single cells are stacked together, and a plate-shaped separator is used as a member that is positioned on the outermost layer of each cell and separates the cells from each other in the stack. The separator functions to supply a fuel gas to an anode side and supply an oxidizing agent to a cathode side, as well as functions to discharge produced water generated inside the cell.
Each cell of a polymer electrolyte fuel cell is a structure in which a gas diffusion layer, a gas flow path, and a separator are each arranged on both sides of a membrane electrode assembly (MEA). Each cell, in which the gas flow path forms a different structure than the separator, has expanded metal as the structure that forms the gas flow path.
The expanded metal is a continuous structure in which hexagonal mesh is arranged staggered. The mesh is arranged so as to form a sloped surface between the gas diffusion layer and the separator, such that gas flow paths are alternately arranged between the staggered mesh and the gas diffusion layer surface and between the staggered mesh and the separator surface. With the expanded metal, mesh is formed by making slits one strand at a time in a flat plate member using a mold while the flat plate member is advanced.
Japanese Patent Application Publication No. 2010-170984 (JP 2010-170984 A) describes a structure in which, in order to reduce gas pressure loss in a gas flow path of a cell, a bond portion that joins expanded metal mesh rises at a position where the bond length is partially reduced and forms part of a strand portion. Also, in an expanded metal manufacturing apparatus, the rising portion is appropriately formed by changing the number of continuations when continuously feeding material in an upper blade direction, i.e., a direction perpendicular to the feeding direction of the material, for each suitable location or region of the expanded metal, by changing shift control logic in the upper blade direction.
With expanded metal in which the mesh is arranged staggered, the gas flow path is arranged between the gas diffusion layer surface and the separator surface, so gas exchange between gas flowing on the gas diffusion layer side and gas flowing on the separator side is possible.
However, near the inlet of oxidizing gas such as air on the cathode side, oxygen has not yet been consumed. Therefore, there is a relatively large amount of gas so the amount of produced water that is carried away is large, and as a result, the area near oxidizing gas inlet tends to become dry. In particular, there is a significant tendency for the area near oxidizing gas inlet to become dry when supplying oxidizing gas such as air in a non-humidified state. Therefore, at high temperatures, the electric power generating performance on the oxidizing gas inlet side decreases and electric power generation becomes concentrated at the gas outlet side, such that the distribution of electric power generation in the electric power generating surface becomes uneven.
It is possible to suppress the evaporation of produced water from the gas diffusion layer by increasing the contact rate or contact area between the expanded metal and the gas diffusion layer. However, although this would enable the electric power generation capability to be maintained at high temperatures, the output voltage may end up decreasing at normal temperatures as a result of an increase in concentration overpotential due to lack of oxygen.